One approach to immunotherapy involves engineering patients' own (or a donor's) immune cells to express cell-surface antigen receptors (CARs) that recognise and attack tumours. Although this approach, called adoptive cell transfer (ACT), has been restricted to small clinical trials so far, treatments using these engineered immune cells have generated some remarkable responses in patients with advanced cancer.
The Chimeric Antigen Receptor (CAR) consists of an antibody-derived targeting domain fused with T-cell signaling domains that, when expressed by a T-cell, endows the T-cell with antigen specificity determined by the targeting domain of the CAR. CARs can potentially redirect the effector functions of a T-cell towards any protein and non-protein target expressed on the cell surface as long as an antibody-based targeting domain is available. This strategy thereby avoids the requirement of antigen processing and presentation by the target cell and is applicable to non-classical T-cell targets like carbohydrates. This circumvention of HLA-restriction means that the CAR T-cell approach can be used as a generic tool broadening the potential of applicability of adoptive T-cell therapy. See, eg, Methods Mol Biol. 2012; 907:645-66. doi: 10.1007/978-1-61779-974-7_36, “Chimeric antigen receptors for T-cell based therapy”, Cheadle E J et al.
The first CAR-T construct was described in a 1989 paper by immunotherapy pioneer Zelig Eshhar in PNAS. The structure of the CAR now comprises a transmembrane polypeptide chain which is a chimaera of different domains from different cellular proteins. For example, the CAR has an extracellular part joined (often by a linker and/or a hinge region) to an intracellular part, with a transmembrane portion of the CAR embedding the receptor in the membrane of an immune cell, normally a T-cell. The extracellular moiety includes an antibody binding site (usually in the form of an scFv, such as derived from a mouse mAb) that recognizes a target antigen, that commonly is a tumour associated antigen (TAA) on the surface of cancer cells. Antigen recognition in this way dispenses with the need to rely on TCRs that require MHC-restricted antigen presentation, and where binding affinities may be relatively low. The intracellular moiety of the CAR typically includes a CD3-zeta (CD3ζ) domain for intracellular signaling when antigen is bound to the extracellular binding site. Later generation CARs also include a further domain that enhances T-cell mediated responses, which often is a 4-1BB (CD137) or CD28 intracellular domain. On encountering the cognate antigen ligand for the CAR binding site, the CAR can activate intracellular signaling and thus activation of the CAR T-cell to enhance tumour cell killing.
Most CAR-Ts expand in vivo so dose titration in a conventional sense is difficult, and in many cases the engineered T-cells appear to be active “forever”—i.e., the observation of on-going B-cell aplasia seen in most of the CD19 CAR-T clinical studies to date. This poses a serious problem for CAR T-cell approaches. Some observed risks are discussed in Discov Med. 2014 November; 18(100):265-71, “Challenges to chimeric antigen receptor (CAR)-T cell therapy for cancer”, Magee M S & Snook A E, which explains that the first serious adverse event following CAR-T cell treatment occurred in a patient with colorectal cancer metastatic to the lung and liver (Morgan et al., 2010). This patient was treated with T cells expressing a third-generation CAR targeting epidermal growth factor receptor 2 (ERBB2, HER2). The CAR contained an scFv derived from the 4D5 antibody (trastuzumab) that is FDA approved for the treatment of HER2-positive breast cancers (Zhao et al., 2009). The patient developed respiratory distress within 15 minutes of receiving a single dose of 1010 CAR-T cells, followed by multiple cardiac arrests over the course of 5 days, eventually leading to death. Serum analysis four hours after treatment revealed marked increases in the cytokines IFNγ, GM-CSF, TNFα, IL-6, and IL-10. CAR-T cells were found in the lung and abdominal and mediastinal lymph nodes, but not in tumour metastases. The investigators attributed toxicity to recognition of HER2 in lung epithelium resulting in inflammatory cytokine release producing pulmonary toxicity and cytokine release syndrome (CRS) causing multi-organ failure (Morgan et al., 2010). Trials utilizing second-generation HER2-targeted CARs derived from a different antibody (FRP5) following conservative dose-escalation strategies are currently underway for a variety of HER2+ malignancies by other investigators (clinicaltrials.gov identifiers NCT01109095, NCT00889954, and NCT00902044).
A variation on the CAR T-cell theme are antibody-coupled T-cell receptor (ACTR) therapeutics, which use CD16A (FCγRIIIA) to bind to Fc regions of tumour-specific IgG (see eg, WO2015/058018, US2015139943). The aim is to enable more control of CAR T-cell activity in vivo by titrating IgG administered to patients. The CD16 binding sites of the CAR-T-cells may be free, however, to also bind to endogenous IgG of the patients and this reduces the attractiveness of the approach. The approach also needs to consider the inherently long half-life of IgG in the body (around 20 days for IgG in man), which may limit control of CAR-cell activity. Ongoing studies may assess the risk of this.
It would be desirable to provide an alternative way to modulate (downregulate or upregulate) immune cell-based therapies, like CAR-T-cell approaches and other cell-based approaches. It would also be desirable to provide a way to address diseases and conditions mediated by endogenous immune cells, such as autoimmune, inflammatory and infectious diseases and conditions.